This invention relates to a vertical deflection method and circuit for use in a personal computer display unit or the like which is capable of automatically correcting vertical deviations in the position of the video picture plane.
When a raster scanning display unit is accessed by a personal computer it is usually operated in a plurality of indicating modes which have different relative timing, for example a coarse display mode of 200 scanning lines and a high resolution mode of 400 scanning lines or more to furnish interchangeability with existing software and to implement new functions.
Further, two vertical deflection frequencies of 50 Hz and 60 Hz are commonly used corresponding to commercially available power supply frequencies. In such situations, when the timing between the video signal and the vertical sync signal differs due to the different indicating modes, the vertical position and width of the video picture plane are changed on the screen of the display tube. Users thus have to manually correct the vertical position and width of the video picture plane. With personal computers this makes setting the indicating mode in which a wide change in timing is required difficult, and limits the conditions for using the different indicating modes.
FIG. 1 shows a circuit diagram of a conventional vertical deflection circuit, wherein an input terminal 60 is connected to a single ended push-pull circuit (SEPP circuit) 30 comprising transistors 1, 2. Transistor 1 is connected to a d.c. supply terminal 61 and transistor 2 is grounded. The output terminal of the SEPP circuit 30, namely the emitter junction between the two transistors, is connected to one end of a vertical deflection coil 3.
The other end of the coil 3 is grounded through a series combination of a bypass capacitor 4 and a pincussion correction circuit (PCC circuit) 5. It is also grounded through a series combination of a resistor 6 and a correction circuit 8 comprising a transistor. The d.c. supply terminal 61 is connected to the junction between resistor 6 and the correction circuit 8 through a resistor 7, and is also grounded through a variable resistor 9. A sawtooth drive signal is applied to the input terminal 60 of the SEPP circuit 30 from an oscillator (not shown). The SEPP circuit amplifies such drive signal and generates a sawtooth a.c. current i.sub.v.
The correction circuit 8 controls the d.c. current I flowing from the d.c. supply terminal 61 through the deflection coil 3, and varies the magnitude of such d.c. current according to the value of the variable resistor 9. The vertical deflection coil 3 deflects the beam of the cathode ray tube to scan the video picture plane in the vertical direction according to deflection current (i.sub.v +I).
The bypass capacitor 4 cuts off the d.c. component of the deflection current (i.sub.v +I). The PCC circuit 5 modulates the horizontal deflection current iH flowing through a secondary coil 52 with the vertical deflection current i.sub.v flowing through a primary coil 51.
It is assumed that the d.c. voltage determined by the value of the variable resistor 9 is E.sub.2 as shown at point A in FIG. 1, and the average voltage of the a.c. current is E.sub.1 as shown at point B. When E.sub.1 &gt;E.sub.2, the d.c. current I flows from the d.c. supply terminal 61 through transistor 1, deflection coil 3, resistor 6, and the transistor in correction circuit 8 to ground. When E.sub.1 &lt;E.sub.2, the d.c. current I flows from the d.c. supply terminal 61 through resistor 7, resistor 6, deflection coil 3, and transistor 2 to ground. This d.c. current I is given by I=(E.sub.2 -E.sub.1)R where R is the value of resistor 6. Accordingly, since the actual deflection current flowing through the coil 3 is i.sub.v +I, when I&gt;0, for example, the vertical position of the video picture plane on the screen of the cathode ray tube is shifted downwardly with respect to the center of the CRT. Conversely, when I&lt;0 the vertical position of the video picture plane is shifted upwardly. The d.c. current I flowing through the deflection coil 3 is thus adjusted by changing the value of the variable resistor 9 to appropriately center the vertical position of the picture plane on the screen of the CRT.
In such a conventional vertical deflection circuit, users thus have to manually correct the vertical position of the video picture plane while viewing the screen of the CRT. Accordingly, such a circuit is not well suited for use in the display unit of a personal computer in which the indicating modes are frequently changed.